1. Field of the Invention
This invention relates generally to oil well tools, and more particularly to down-hole measurement tools.
2. Description of the Related Art
In the oil and gas industry, hydrocarbons are recovered from formations containing oil and gas by drilling a well borehole into the formation using a drilling system. The system typically comprises a drill bit carried at an end of a drill string. The drill string is comprised of a tubing which may be drill pipe made of jointed sections or a continuous coiled tubing and a drilling assembly that has a drill bit at its bottom end. The drilling assembly is attached to the bottom end of the tubing. To drill a borehole, a mud motor carried by the drilling assembly rotates the drill bit, or the bit is coupled to drill pipe, which is rotated by surface motors. A drilling fluid, also referred to as mud, is pumped under pressure from a source at the surface (mud pit) through the tubing. The mud serves a variety of purposes. It is designed to provide the hydrostatic pressure that is greater than the formation pressure to avoid blowouts. The mud drives the drilling motor (when used) and it also provides lubrication to various elements of the drill string. The mud is also used in many systems as a signal transmission medium using a transmission method known as mud-pulse telemetry.
It is often desirable to gather information of a specific formation once the borehole reaches an area known in the art as the zone of interest. At the zone of interest, down-hole instruments and/or sampling devices are utilized to gather data regarding various parameters of interest including pressure, temperature and other physical and chemical properties of the formation fluid and or mud. The down-hole operations are known as measurement while drilling (MWD) or logging while drilling (LWD).
One MWD method used to determine characteristics of formation fluid is known as nuclear magnetic resonance or NMR well logging. NMR well logging instruments can be used for determining properties of earth formations including the fractional volume of pore space and the fractional volume of mobile fluid filling the pore spaces of the earth formations.
In NMR tools, a magnet is used to produce a static magnetic field in the formation. The static field aligns the nuclear spins within the formation. An RF field is applied to realign nuclear spins generally perpendicular to the static field. At the end of the RF pulse, the nuclear spins precess back towards alignment with the static field. Signals generated from the precessing spins are picked up by a receiver. The tools use an antenna for creating the RF field and for receiving the echo signal from the formation fluid being analyzed. High gain amplifiers are utilized to amplify the received echo prior to processing the signal, but it is very important that the echo is distinguishable over other signals known as noise.
A major problem with NMR testing relates to tool movement. A typical pulse NMR measurement is sensitive to movement such as vibration, axial, horizontal and rotational displacements with respect to the formation. These types of movement during a test may induce noise in the system, and will deteriorate the NMR result, sometimes to the extent that test data is invalidated.
The present invention addresses the problems encountered when testing formations with movement-sensitive instruments during drilling operations. The invention provides an apparatus and method to minimize vibrations caused by drilling operations at during periods of testing.
The present invention provides an apparatus for MWD comprising drill string with a sensor mounted on the drill string for sensing a parameter of interest of a formation. A clocked controller is disposed on the drill string for controlling timing of the NMR data acquisition; and a second clocked controller is disposed at a surface location. During drilling operations, the two clocked controllers are initially synchronized such that the surface controller is performing certain tasks in timed sequence with the sensor even though the surface controller is not connected to the sensor.
The present invention also provides a method for MWD comprising conveying a drill string into a well borehole and sensing a parameter of interest of a formation traversed by the borehole with a sensor mounted on the drill string. The method also includes controlling timing of the sensor with a first clocked controller disposed on the drill string, and synchronizing a second clocked controller disposed at a surface location with the first clocked controller.
The benefits accorded by the present invention are, among others, cost savings and drilling efficiency. The cost savings are realized when expensive testing is not repeated due to data corruption by unnecessary movement. The drilling operations are more efficient, because drilling is only halted for the brief amount of time a test is performed. Drilling is resumed at the precise moment a test is complete.